The Influence of Microstructure on High Temperature Properties of 2024 Alloy

A commercial 2024 aluminum alloy and a modified 2024 alloy containing Zr and V were subjected to a variety of thermomechanical processing (TMP) treatments to produce different grain structures, dislocation substructures and precipitate distributions.

Studies were made of the effects of microstructural variations on tensile deformation from room temperature to 450°C, and creep behavior at l50°C. It was found that materials with structures containing both lathe-shaped S precipitates and a dislocation substructure had the highest tensile strength to 250°C. The highest creep strength was observed in materials which had lathe-shaped S precipitates and only a nominal dislocation substructure.

Materials whose microstructures contained a dislocation substructure and spheroidized S precipitates had the lowest creep strengths. The stress dependence of the minimum creep rate did not obey a simple power law behavior. Both alloys having various TMP treatments shoved a decreasing stress dependence of the creep rate at low stresses.

In the last decade there have been numerous studies concerned with thermomechanical processing (TMP) of 7000 series (Al-Zn-Mg-Cu) and 2000 series (Al-Cu-Mg) aluminum alloys. By inducing microstructural changes through TMP and composition control, researchers have attempted to improve a variety of properties: resistance to stress corrosion, fracture toughness, yield strength, fatigue strength, and creep strength.

This article describes changes of tensile and creep deformation of a commercial 2024 aluminum alloy and modified 2024 alloy after subjecting to a variety of TMP treatments.

Chemical analyses of the two experimental alloys-Commercial 2024 and Modified 2024 Alloys are given in the Table I.

	Cu (%)	Mg (%)	Mn (%)	Fe (%)	Si (%)	Ti (%)	Zr (%)	V (%)	Al
Commercial 2024	4.8	1.4	0.59	0.31	0.19	0.04	-	-	Rest.
Modified 2024	4.7	1.4	0.49	0.27	0.17	0.04	0.15	0.10	Rest.

The compositions were chosen to vary the intermediate-size insoluble dispersoids. The commercial 2024 alloy contains Mn which results in formation of stable ellipsoidal Mn-rich dispersoids, approximately 50 to 200 µm in size.

The modified alloy contains Zr and V in addition to Mn, which leads to both the Mn-rich dispersoids and finer stable particles of A1₃Zr approximately 10 to 70 µm in diameter. These dispersoids are normally precipitated during ingot homogenization, and the size is relatively unchanged by primary working.

Tensile deformation from room temperature to 450°C was performed in an Instron testing machine at a strain rate of 1.67 x 10^-4 sec^-1 (0.01 min^-l). Constant stress creep tests were conducted in air at 150°C. Minimum creep rates were determined as a function of the applied stress and the microstructural variations induced by different TMP treatments. Both tensile and creep specimens were machined with their axes parallel to the rolling direction.

Microstructures. Four TMP conditions were used to develop different microstructures-the grain structure, dislocation substructure, and precipitate distribution and morphology.

TMP to condition A (Table II) involved simply heat treating the rolled plate to the T6 condition. This resulted in a grain structure elongated in the rolling direction for both alloys, and an absence of dislocation substructure. The final aging treatment precipitated the usual lathe-shaped S (Al₂CuMg), and in a number of instances there were precipitate- free zones along the grain boundaries.

Condition B consisted of solution annealing plus quenching, a light pre-age followed by cold rolling, and a final age.

Condition C involved solution treating and quenching, pre-aging, two cycles of 10% cold rolling with intermediate aging, and a final age at 200°C. This TMP resulted in an elongated grain structure and very fine S precip1tates superimposed on a tangled dislocation cell structure.

Condition D consisted of solution annealing and quenching followed by two cycles of 10% cold rolling followed by aging. In this condition, the alloys had an elongated grain structure and contained a very dense tangled dislocation structure with no visible S precipitates, i.e., the structure was underaged.

Table II. Various Microstructures Produced in 2024 Alloys by TMP

Designation of Alloy Condition	Thermomechanical Processing Treatment(a)	Microstructure
Commercial 2024 (Contains Mn)
A	T6 condition; SHT 505°C 2.5 hrs, W.Q., aged 16 hrs at 190°C	Lathe shaped S, Mn-rich insoluble dispersoids, coarse Fe-Si particles, absence of dislocation, substructure, elongated grain structure
B(b)	Solution anneal 505°C, 2.5 hrs, W.Q., aged 1 hr at 290°C, C.R. 75% at RT, aged 16 hrs at 170°C	Aligned spheroidal and slightly elongated particles(d); Mn-rich dispersoids, coarse Fe-Si particles, tangled dislocation cell structure, cold worked, optical microstructure
C(c)	Solution annealed 505°C 2.5 hrs, W.Q., aged 1 hr at 200°C, C.R. 10% at RT, aged 3 hrs at 200°C, C.R. 10% at RT aged 3 hrs at 200°C	Some fine S, Mn-rich insoluble dispersoids, coarse Fe-Si particles, tangled dislocation cell structure, elongated grain structure
D(d)	Solution annealed 505°C 2.5 hrs, W.Q., C.R. 10% at RT, aged 3 hrs at 150°C, C.R. 10% at RT aged 3 hrs at 160°C	Under aged-no detectable S, Mn- rich insoluble dispersoids, coarse Fe-Si particles, tangled dislocation cell substructure, elongated grain structure
Modified 2024 (Contains Mn. Zr.V)
A	T6 condition, same as commercial alloy, Condition A	Same as commercial alloy, Condition A--except contains fine Al3Zr dispersoids
B(b)	Same as commercial alloy, Condition B	Same as commercial alloy, Condition B--except contains fine Al3Zr dispersoids
C(c)	Same as commercial alloy, Condition C	Same as commercial alloy, Condition C--except contains fine Al3Zr dispersoids
D(d)	Same as commercial alloy, Condition D	Same as commercial alloy, Condition D--except contains fine Al3Zr dispersoids

(a) Symbol definition:
        SHT = solution heat treat
        W.Q. = water quench
        RT = room temperature
        C.R. = cold roll.
(b) Thermomechanical processing treatment similar to that of Pattanaik, et al
(c) Thermomechanical processing treatment due to Lipsitt
(d) These particles could have resulted from breaking up S lathes during cold working followed by some spheroidization during aging.

It is difficult to assess unequivocally the relative contributions of dislocation substructure and precipitation hardening to the creep strength because the substructures and dispersions often differ in character, and the tradeoffs in strength are difficult to pinpoint.

However, the various effects can be discussed in general terms. A summary of the observations is as follows:

The combination of a tangled dislocation substructure and a fine precipitate of S lathes or C.P.B. zones gave the highest tensile strengths up to 250°C.

The combination of small, approximately spherical particles (probably spherodized S particles), and a tangled dislocation structure had the lowest tensile strength up to 350°C and the lowest creep strength at l50°C.

The materials containing a dislocation substructure plus fine S or C.P.B. zones were weaker during creep at 150°C than material containing only S, except in the presence of a fine Al₃Zr precipitate.

The under aged condition had the highest tensile strength at 100 to 200°C.

The stress dependence of the creep rate of both alloys was similar and no simple power law was observed.

Both alloys, for Conditions A, C, and D, showed a decreasing stress dependence of the creep rate at low stresses.

Conclusions

1.   Dislocation substructure strengthening in combination with precipitation hardening is viable for increasing short-time, tensile strength up to 250°C in 2024-type alloys, but the S and C.P.B. zones are more potent hardeners than is a dislocation substructure.

2.   Dislocation substructures usually degrade the creep strength at 150°C. At best, substructures have no effect.

3.   No simple power low describes the creep behavior at 150°C; the stress dependence of the creep rate decreases from 16 to 20 at high stresses to 3 to 4 at low stresses. This is probably due to a change in creep mechanism for dislocation.